1. Field of the Invention
This invention relates to a reactive matrix and a process for removing impurities from a fluorine containing gas. More particularly this invention relates to a reactive matrix and a process for removing moisture from nitrogen trifluoride gas or fluorine gas while avoiding undesirable exothermic reactions.
2. Description of Prior Art
At the present time, a wide array of supports for a reactive matrix are utilized to remove impurities from nitrogen trifluoride gas. A major problem with presently available reactive matrices is that some of the impurities in the nitrogen trifluoride such as N.sub.2 F.sub.2 and N.sub.2 F.sub.4 are not compatible with some of the supports presently used. These impurities decompose exothermically to catalyze the decomposition of NF.sub.3 which, in turn, causes a cascade of reactions that results in undesirably high temperatures which may even destroy the housing utilized for the reactive matrix. It has been observed by Broer et al, J. Mater. Res., 1988, 3(4), 755 that reaction of components of NF.sub.3 gas with alumina did not occur until 650.degree. C. The reaction of nitrogen trifluoride with alumina at temperatures greater than 650.degree. C. were shown by FTIR and subsequent analysis to produce nitrosyl fluoride (NOF), NO.sub.2, NO and AlF.sub.3.
It also has been shown in Japanese patent 01261209 (1989) that the purification of NF.sub.3 with previously dehydrated alumina within a temperature range of 0 to -125.degree. C. removes impurities of N.sub.2 O, CO.sub.2 and N.sub.2 F.sub.2. The cold temperature minimized the adsorption of NF.sub.3 onto the alumina support.
Japanese patent 0203450 A2 (1990) discloses that purification of NF.sub.3 in stages prevents any temperature increases and subsequent explosions. N.sub.2 F.sub.2 and N.sub.2 F.sub.4 are first removed by passing the NF.sub.3 gas through a metallic tube, e.g. stainless steel or Monel, at 150-300.degree. C. Subsequently, OF.sub.2 is removed by bubbling the gas through an aqueous solution containing Na.sub.2 SO.sub.3 and Na.sub.2 S. The purified NF.sub.3 then flows through a previously activated alumina layer for removal of N.sub.2 O and CO.sub.2. Both of these impurities are present in concentrations higher than water.
U.S. Pat. Nos. 4,853,148 and 4,925,646 disclose the use of aluminum trifluoride on an alumina support to remove moisture from a hydrogen halide gas. The aluminum trifluoride is formed by reaction of corresponding partially or fully alkylated compounds and/or pendant functional groups with the corresponding gaseous hydrogen halide. The partially or fully alkylated compounds are utilized as a solution in an organic solvent. Formation of aluminum trifluoride or aluminum by this process results in trace amounts of organic moieties or compounds being present on the resultant product. Even trace amounts of organics on a material used to purify NF.sub.3 are undesirable since they react with NF.sub.3 or impurities in NF.sub.3 in an exothermic reaction which generates sufficient heat to cause further undesirable reaction of the NF.sub.3.
In addition, formation of AlF.sub.3 via reaction of the starting material, AlR.sub.3, where R is an organic moiety, is not practical with gaseous HF since gaseous HF has a low vapor pressure and excess unreacted HF is difficult to remove from the support. Furthermore, formation of AlF.sub.3 from aluminum hydride and HF gas cannot be done with HF in aqueous solution since water reacts with aluminum hydride.
Accordingly, it would be desirable to provide a scavenger for moisture in NF.sub.3 which does not react with NF.sub.3 or with impurities normally found in NF.sub.3. In addition, it would be desirable to provide such a scavenger which does not react with compounds which are adsorbed by the scavenger.